CO2-Assisted Li–O2 Batteries: Challenges and Strategies Towards Practicality

Abstract

Today, safe alternatives to lithium–ion batteries able to answer ever-increasing energy demands require an urgent focus. Amongst promising candidates, the environmentally-friendly conversion-type lithium–oxygen (Li–O2) battery offers high theoretical energy density by coupling a Li metal anode with an “air-breathing” cathode. Since its inception, however, the Li–O2 cell has remained plagued by the poor safety characteristics of the metal anode and the sluggish O2 electrochemistry at the cathode side, leading to poor round-trip efficiencies and unpractical lifetimes. Most critically, efforts to shift from pure O2 to practical ambient air conditions have long revealed that CO2 and moisture in the air radically affect the O2 electrochemistry, the resulting product selectivity, and the overall performance of these devices. In a tetraglyme-based electrolyte, our team demonstrated that CO2 shifts the discharge reaction mechanism toward the formation of Li2CO3 (with complete Li2O2 absence), which requires unpractical potentials (>4.5 V) upon cell recharge. In this talk, efforts to shed light on the mechanism of these devices and uncover strategies for performance enhancement will be discussed. We recently probed the possibility of tuning the battery cycling conditions/operating temperatures and the solvation properties of chosen solvents/salts as promising strategies to stabilize soluble peroxocarbonate reaction intermediates with lower oxidation potentials. Alternatively, our team has also shed light on the promise of integrating suitable redox mediators to facilitate the decomposition of Li2CO3 in this CO2-assisted Li–O2. A correlation between the required concentration of redox species and the morphology/crystallinity of discharge products and the promise of coupling for the first time electrochemical/spectroscopic analyses to uncover the nature of these operating redox species (e.g., Br2···Br3 − anionic complexes) will be further discussed during this talk.